This invention relates to a system for sensing ionic components. More particularly, the invention relates to sensors, and methods for making and using sensors, useful in sensing ionic components, e.g., hydrogen ions or hydroxyl ions-measured by pH, in fluids, such as blood.
It is often advantageous to determine the concentration of an ionic component in a given fluid. For example, medical diagnostic and/or treatment procedures may involve the determination of the pH value of a patient's blood or other bodily fluids. Such determinations may be made very frequently, even continuously, during the procedure.
For biological fluids, a prior known sensor uses the fluorescent properties of a dye in conjunction with the ionic permeability of a preformed integral cellulose membrane sheet. In this sensor, the cellulose membrane is chemically treated so as to introduce covalent bondable groups onto the membrane. A fluorescent dye, suitable for providing a signal which varies as the concentration of the ionic component of interest varies, is then covalently bonded to these groups to adhere the dye to the membrane. A small disc is cut from the membrane sheet and is placed in a well of a sensor cassette, which itself is placed in proximity to an optical fiber. An opaque overcoat is physically placed over the exposed surface of the disc and is secured, e.g., heat staked, to the cassette. This overcoat, which is physically separate from the disc provides optical isolation for the dye in the disc. When the dye is excited by excitation light imposed on the dye, it undergoes fluorescence, emitting a light signal. This emission light signal is transmitted, by the optical fiber, to a processor where it is analyzed to provide a determination of the concentration of the ionic component of interest.
One problem which exists with such membrane-type sensors relates to response time. Such sensors are relatively slow to respond to changes in ionic component concentration. Sensor response time is particularly important in situations where a patient's blood is frequently, or even continuously, monitored and the information obtained from such monitoring is used as a basis for treating the patient. The separate opaque overcoat may contribute to the reduced response time of such sensors, and may cause a variability in the response so that unreliable signals are provided. Further, the separate overcoat has a disadvantageous tendency to be displaced during use, and the heat staking technique can compromise the sterility of the blood flow path.
A faster, more reliable and durable sensor for ionic components, in particular hydrogen ions or hydroxyl ions-measured by pH, would be advantageous.